(1) Field of the Invention
The present invention relates to an improved fairlead for a tow cable handling system and more particularly to a fairlead that protects the handling system components even when the tow cable breaking strength exceeds the rated strength of the handling system.
(2) Description of the Prior Art
It is known in the art that ships employing tow cables require a cable handling system that is rated for a failure load higher than the breaking strength of the cable being used. This is because, in the unlikely event that a tow cable snags, cable tension can quickly approach the breaking strength of the cable. Ideally, an overstressed tow cable will break at mid-span and not tax the handling system. A less desirable alternative is a case where a tow cable does not reach breaking strength because the winch shaft, the winch foundation or the fairlead foundation of the handling system first fails due to excess cable tension load.
Cable diameter is a major factor in towed sonar system design. A small diameter tow cable (i.e., less than 0.75″ diameter) usually cannot achieve the depth needed for surface ship towed sonar system operations. A larger diameter cable will tow at an increased depth because as the cable diameter increases, the weight of the cable in the water increases faster than the hydrodynamic drag of the cable. However, the breaking strength of the cable, and the corresponding handling system load rating requirements, also increase with cable diameter. For example, a typical 1″ diameter tow cable with a cross-sectional area of 0.7854 int typically has a breaking strength of 40,000 lb or more. This breaking strength often requires a winch rated at 60,000 lb or greater in order to include an acceptable factor of safety, 1.5 in this case. The supporting deck and underlying foundation also need to have the same or a higher load rating.
Attempting to retrofit a 1″ diameter cable to a legacy handling system designed for a 0.8″ diameter cable in order to permit greater tow depth can create problems—even if the handling system winch has enough volume to accommodate the new larger cable. A typical 0.8″ diameter cable has a breaking strength of 25,600 lb. The handling system for the 0.8″ diameter cable would be designed to incorporate a typical breaking strength 1.5 factor of safety load rating of 38,400 lb. The 40,000 lb breaking strength of the new 1″ diameter cable will exceed this handling system rating which in most cases is fixed and cannot be changed, thus risking catastrophic failure of the handling system upon the occurrence of a cable snag.
One known approach to overcome the excessively high cable breaking strength problem involves simply letting the cable unreel freely from the winch if a cable snag occurs. Unfortunately, with this approach, the rotational inertia of the typical winch usually prevents the winch from speeding up fast enough to relieve the momentary tow cable tension surge. The time required to unlock the winch or release the brake is also non-negligible, and would require automated detection of any tension surges in order to preclude a delayed response.
Other known approaches to resolving the tension forces caused by a snag are to use a guillotine-type mechanism to quickly cut the cable mechanically or to employ a cutting torch. However, a typical tension surge occurs so quickly (typically in a 200 millisecond spike) that the response time of the cutter or torch system may not be fast enough. The tension surge duration is related to the difference in speed between the ship and the relatively immovable object snagging the cable, the cable length, and the cable stiffness. Any change in cable tension propagates along the cable at 5000 meters per second.
Guillotine mechanisms have been used previously in conjunction with helicopter-deployed dipping sonar systems that employ long synthetic cables of small diameter (typically 0.5″ or less). Several factors make a guillotine approach feasible for such dipping sonar systems: (1) synthetic cables stretch more than steel cables before breaking; (2) a small diameter cable with low stiffness exhibits more stretch; (3) a long cable length is employed with dipping sonar systems increasing total stretch, and (4) the essentially stationary helicopter deploying the system allows both the stretch and the tension to increase slowly in the event of a cable snag.
Another common approach is to employ a mechanical fuse-like weak link that is incorporated into the tow cable and is designed to break at a tension lower than the breaking strength of the cable. It is a better approach than using a guillotine type device or depending on an unreeling winch, but a snag could still potentially occur above the fuse placement.
Whatever object snags the tow cable, the object will tend to slide backward along the cable, stop, and then generate a tension surge from that point back to the cable handling system on board the tow ship. The most likely snag stopping point along the cable is the distal end of the tow cable. What is needed is a rapid means to prevent the cable tension surge from damaging the tow cable handling system components.